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RSC-Advances 2014-4-16298 by SuryadiIsmadji

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42 < 1% match (student papers from 16-Apr-2014)Submitted to Universiti Sains Malaysia on 2014-04-16

paper text:RSC Advances PAPER

6Antibiotic detoxification from synthetic and real

Cite this: RSC Adv., 2014, 4, 16298

4effluents using a novel MTAB surfactant- montmorillonite (organoclay)sorbent† Merry Anggraini, a Alfin Kurniawan, a Lu Ki Ong, a Mario A. Martin,a Jhy-Chern Liu, b Felycia E. Soetaredjo, a Nani Indraswatia and SuryadiIsmadji*

a The growing threat of antibiotic-resistant bacteria to public health has raised interest in the propertreatment of discharged pharmaceutical wastewater before entering surface waters. In this study, adsorptionwas highlighted as a low cost and effective pathway to remove

25amoxicillin and ampicillin from aqueous solutions. Montmorillonite (Na-MMT) and myristyltrimethylammonium (MTA)-intercalated montmorillonite (O-MMT) were employed as the adsorbing solids.

Static adsorption experiments were performed

17at three temperatures (303.15 K, 313.15 K and 323.15 K)

for single antibiotic systems. The adsorption isotherm curves at all temperatures exhibited an L2-typeisotherm. The Freundlich and Langmuir models were applied to analyze single adsorption isotherm data.The maximum sorption capacity of 0.124–0.133

29mmol g 1 for amoxicillin and 0. 143–0.157 mmol g 1 for

ampicillin was Received 13th January 2014 estimated for O-MMT sorbent from Langmuir fitting. A modifiedextended-Langmuir model with the Accepted 20th March 2014 inclusion of surface coverage (q) wasproposed for analysis of




40binary adsorption isotherm data. The fitness of the

modified extended-Langmuir model was superior to the original model. Batch adsorption DOI:10.1039/c4ra00328d tests on real pharmaceutical wastewater demonstrated the feasibility of the O-MMTsorbent for practical www.rsc.org/advances applications. Introduction Currently, a large group of antibiotics isavailable in the market and they

35have been proven to be powerful drugs to treat various bacterialinfections, from minor to life- threatening ones.

Anti- biotics are generally produced by or derived from microorgan- isms such as fungi or bacteria and theycan also be chemically synthesized and particular examples are penicillins, cephalo- sporins, macrolides,rifamycins, sulfonamides, chloramphenicol, tetracyclines and aminoglycosides. Of particular interest arepenicillin groups including amoxicillin and ampicillin. Amoxi- cillin is a moderate-spectrum of b-lactamantibiotics and is usually the drug of choice within penicillin groups due to its better absorptivity,

24following oral administration than other b-lactam antibiotics. Amoxicillin iswidely

24used in the treatment of a number of bacterial infections

include pneumonia, bron- chitis, laryngitis, gonorrhea, skin and urinary tract infections.1 Ampicillin is also ab-lactam antibiotic, part of the aDepartment

2of Chemical Engineering, Widya Mandala Surabaya Catholic University,Kalijudan 37, Surabaya 60114, Indonesia. E-mail: suryadiismadji @yahoo.com;Fax: +62 31 389 1267; Tel: +62 31 389 1264 bDepartment of ChemicalEngineering, National Taiwan University of Science and Technology, No. 43,Sec. 4, Keelung Rd., Taipei City 106, Taiwan, Republic of China

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra00328d aminopenicillinfamily, which is closely related to amoxicillin in terms of spectrum and activity level.2 Both amoxicillin andampicillin work in a similar manner against Gram Positive and Gram Negative bacteria by interfering cell wallsynthesis so that the human antibodies can penetrate and remove them.2 In spite of its usefulness andvaluable contributions in human therapy, antibiotic-resistant bacteria are the today's most pressing clin- icaland public health concerns that continue to grow due to abuse and overuse of antibiotics. This leads toconsequent treatment complications and increased healthcare costs because the target bacteria arebecoming more resistant to the exposure of therapeutic levels of an antibiotic. The dissemination of



antibiotics in natural environments (e.g., lakes and streams) may come from various sources, such ashuman and animal excretion, agriculture, aquaculture and livestock farming, hospital sewage sludge anddiverse industrial routes.3 The relative concentrations of antibiotics in the industrial effluents are severalorder of magnitude higher than those released from veterinary and hospital sources.3 Although most ofpharmaceutical products administered are particularly designed to have a short half-life, some antimicrobialslike tetracycline, erythromycin, sulfamethoxazole and penicilloyl groups are persistence and poorlymetabolized hence they are only partially eliminated in the sewage treatment plants. The enrichment ofantibiotic residues and their transformed products into receiving waters is worrying as it can impact thestructure and activity of microbiota, spreading of antibiotic- resistant genes to pathogenic bacteria strainsthat can reach humans through food chains and ultimately in the water reuse scenario.4–6 Severaltreatment methods have been developed so far for puri?cation of antibiotic-bearing effluents such as aerobicand anaerobic biological treatments,7,8 advanced chemical oxidation,9–11 membrane separation,12chlorination,13 photocatalysis,14–16 electro-oxidation,17 and adsorption.18–21 Amongst them, adsorptionis considered as the most reliable method for removal of toxic micropollutants from municipal water andwastewater. Regardless of the adsorbent material and system design, adsorption process is generallysimple, adaptable, economically viable and highly effective across a wide range of concentrations,highlighting its advantages over other technologies. The treatment of water and wastewater containingantibiotics by adsorption is the key to modulating the extent of environmental occurrence, transport and fateof this micropollutant. Clays and clay minerals have found potential applications for sustaining theenvironment because they exhibit large adsorption capacity, excellent mechanical and chemical stability,cheap and readily obtained in large quantities. So, efforts toward their applications as a sorbent to abateantibiotics from water and wastewater have been stimulated. Over the past few years, a progressingresearch on the removal of antibiotics using clays or clay minerals can be found in literatures.18–21 In most,if not all, of these studies, single adsorption systems are emphasized and only limited studies dedicate toinvestigate binary or multicomponent systems. In the real pharmaceutical sewage treatment units, two ormore unmetabolized antibiotics may co-exist, thus it is necessary to study binary or multicomponent adsorp-tion equilibria and thermodynamics for effective design and optimization of antibiotic/clay wastewatertreatment systems. To ?ll this gap, the goals of this study are (1) to synthesize a novel organoclay sorbentand (2) to evaluate the performance of pristine and as-synthesized organoclay to remove amoxicillin andampicillin from single and binary (two components) aqueous systems. As far as we are aware, this is the ?rst study demon- strating binary adsorption of amoxicillin and ampicillin using pristine andmyristyltrimethylammonium cation-intercalated (organo) montmorillonite with special attentions to adsorptionequilibria and thermodynamic aspects. We propose a modi?- cation on the extended-Langmuir model byintroducing surface coverage for analyzing binary adsorption equilibrium data. Ultimately, batch adsorptiontests on real pharmaceutical wastewater are demonstrated, along with regenerability evaluation of the claysorbent. Experimental section Chemicals Antibiotics used in this study (i.e., amoxicillin trihydrate andampicillin trihydrate) were kindly provided by a local pharmaceutical industry with minimum purity of 97%and 95%, respectively. The molecular structure and some speci?c information about these compoundsinclude their environmental persistence data22,23 are presented in Table 1. Analytical-grade chemicalsinclude myristyltrimethylammonium bromide (MTAB) cationic surfactant (99%), hydrogen peroxide solution(30%), sodium chloride (99.5%), hydrochloric acid (37%), silver nitrate (99.8%) and potassium hydroxide(85%) were purchased from Sigma-Aldrich, Singapore and used as-supplied.

15Double distilled water (DDW) was used throughout the experiments.

Preparation of



adsorbent materials Montmorillonite lumps as the starting material were collected from one of mining siteslocated at Pacitan town, East Java. A?er the collection, the solid

6was repeatedly washed with tap water to remove coarse particles and water-

soluble impurities. Then, the solid was dispersed in

dilute hydrogen peroxide

31solution with a solid/solution ratio of 1 : 10

(w/v) and the suspension was aged for 2 h under mechanical stirring at 500 rpm. The solid was washed withdouble distilled water and 0.1 N NaOH solution alternately

28until the pH of the washing solution was near -neutral. The clay material wasoven -dried at

383.15 K and stored in an airtight plastic bag for further characterizations. The mineralogical analysis ofclay material was conducted based on the size fractionations method24 and the results are: 72% smectite,4% quartz, 12% feldspar, 8% calcite, 3% anatase and 1% others. To enhance the monoionic nature of clay,??y grams of clay was treated with 250 mL 1 N NaCl solution (?ve cycles stirred at 500 rpm for an hour ineach cycle) and washed until negative reaction of chloride ions with silver nitrate was obtained. The resultingclay (denoted as Na-MMT) was oven

37-dried at 373 .15 K for 24 h, pulverized and

screened with 100/120 sieves to obtain size fractions of 0.125–0.150 mm. The

6cation exchange capacity (CEC) of Na-MMT was

74.2 m eq./100 g of clay, as measured by

6methylene blue index following ASTM C837-99 test method. The

metal oxide compositions of Na-MMT were analyzed using

6a PANalytical MiniPal QC energy dispersive X-ray ?uorescence (EDXRF)spectrometer



and the results are shown as follows: SiO2 of 61.28%; Al2O3 of 18.33%; Na2O of 2.47%, K2O of 1.75%,MgO of 2.16%, CaO of 1.59%, MnO of 0.27%, Fe2O3 of 3.35%, and TiO2 of 0.08%. The preparationprocedure of organo-montmorillonite (designated as O-MMT) was described

11as follows: 10 g Na -MMT was dispersed in 100 mL MTAB solution with a

surfactant

9concentration equivalent to 1.5 times of the CEC of clay. The suspension was

aged for 1 h under stirring at 500 rpm. Then, it was placed in an Inextron WDS900DSL23-2 microwave ovenand irradiated over 5 min at a frequency of 2.45 GHz and an output power of 500 W. The resulting solid waswashed with double distilled water several times until it was free from bromide anions (tested by titration with0.1 M AgNO3 solution). The product was dried in an air-circulating oven at 383.15 K to constant weight,pulverized and sieved. The CEC of O-MMT was 14.9 m eq./100 g of clay according to methylene blueadsorption index. Characterizations of adsorbent materials Scanning electron microscopy (SEM) wasperformed to probe the microtopography and surface

2texture of the adsorbents. The scanning was conducted on a JEOL JSM-6390?eld emission

Table 1 Molecular structure and some specific information about amoxicillin and ampicillin AmoxicillinAmpicillin Molecular structure Physical state Molar mass (g mol 1) Chemical formula Water solubility, 25 C (gL 1) pKa Environmental persistence data Photodegradation rate (s 1) Directa Hydrolysis rate (s 1)b White tooff-white solid 365.4 C16H19N3O5S 3.43 2.4 (carboxylic) 7.4 (amine) 9.6 (phenol) White to off-white solid349.4 C16H19N3O4S 10.1 2.7 (carboxylic), 7.3 (amine) 5.24 10 7 Not available 4.45 10 7 2.15 10 7 a Directphotodegradation at pH 7 using a solar simulator system. b Hydrolysis at pH 7 and room temperature(298.15 K); the hydrolysis rate constants correspond to half-lives of 18 d for amoxicillin and 36 d forampicillin. SEM at an accelerating voltage of 20 kV. Surface characterizations were conducted by physicaladsorption–desorption isotherms of N2 at 77.15 K, on a Micromeritics ASAP 2010 automated sorptometer.The samples were vacuum-outgassed under a ?ow of pure helium at 10 3 Torr and

2473.15 K for 24 h. The speci?c surface area,

micropore volume (Vmic) and external (mesoporous) surface area (Sext) was determined from theadsorption branches applying the

12Brunauer–Emmett–Teller (BET) and t-plot method, respectively. The pore sizedistribution was derived from desorption data by means of Barrett–Joyner–


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Halenda (BJH) method. Total pore volume (VT) was estimated from the volume

of gas adsorbed at a relative pressure (p/ p ) of 0.

99. The pH of point of zero charge (pHpzc) was determined by pH-dri? technique following Yang et al.25method. Brie?y,

4a solution of 0.005 M CaCl2 was boiled to remove dissolved CO2 and thencooled to room temperature.

A 20 mL aliquot of the solution was poured into a series of capped vials. The pH was adjusted by adding 0.5M HCl or 0.5 M NaOH solution to a value between 2 and 11. A known amount of Na-MMT or O-MMT ( 0.05g) was added and the suspension was

4equilibrated for 24 h. The ?nal pH was measured

using a SevenEasy™ digital pH-meter (Model S20, Mettler Toledo) and plotted against the initial pH. The pHat which the curve of pH?nal versus pHinitial crosses the line pHinitial ¼ pH?nal is marked as pHpzc. Theresults are 5.82 for pHpzc of Na-MMT and 7.18 for pHpzc of O- MMT. Thermal decomposition analysis wasperformed on a Mettler- Toledo TGA/DSC 1 thermogravimetric analyzer. Approximately

1310 mg of the samples was spread uniformly at the bottom of aluminacrucible. The

temperature of furnace was programmed to rise from room temperature to a ?nal temperature of 1123.15 Kat

1320 K min 1 in a dynamic high-purity ?owing N2 of 100 mL min 1. Theelemental contents of

Na-MMT and O-MMT materials were determined using an automated CHNS/O elemental analyzer (Model2400-II, PerkinElmer). FT-IR analysis was carried out on a Shimadzu FTIR

98400S spectrometer using KBr disk technique. The spectra data were

collected by accu- mulating 200 scans over wavenumber range of 4000–500 cm 1 in the transmission modeat a spectral resolution of 4 cm 1.



5Data processing includes baseline adjustment, normalization and spectralsmoothing was performed using IRsolution so?ware (Version 1.21). Themineralogical compositions of the

solids were analyzed using

5a Philips PANalytical X'Pert X-ray diffrac- tometer. The

5powder diffractograms of the specimens were acquired at 40 kV and 30 mA

in the range of 2-theta angles of 2– 70

5with a scanning speed of 1 /min. The radiation source was Ni-? ltered Cu Ka1

(l ¼ 0.15405 nm).

Batch adsorption experiments – single solute systems Fresh antibiotic effluents were prepared by dissolving0.3 g amoxicillin or ampicillin into 1 L deionized water to give an

27initial concentration of 0.80 mmol L 1 for amoxicillin and 0. 82 mmol L 1

for ampicillin. For the adsorption equilibrium experiments, the stock effluents of amoxicillin or ampicillin werepoured

37into a series of stoppered conical ?asks (each of 100 mL)

containing Na-MMT or O-MMT with varying doses (0.1–1 g). The ?asks were wrapped with aluminium foil toeliminate light interference. Then, the ?asks were

16placed in a thermostated reciprocal shaker and shaken at 100 rpm for

24 h.

30Preliminary experiments indicated that 24 h provided sufficient time toreach equilibrium.



The system temperature was held constant

17at 303.15 K, 313.15 K and 323.15 K by a built -in

PID-type temperature controller. A?er equilibration, the clay suspension

11was centrifuged at 3000 rpm for 10 min and the supernatant was

taken for analysis. The residual concentration of solute was quanti?ed by a double beam UV-Visspectrophotometer

32at a detection wavelength of 252.2 nm for amoxicillin and 245.8 nm for

ampicillin. The calibration curves were prepared from a set of ?ve standard solutions with concentrationrange of 50–300 mg L 1. Prior to spectrophotometric measurements, all superna- tants were ?ltered througha 0.45 mm syringe ?lter. The amount of solute adsorbed per unit mass of adsorbent

38at equilibrium (qe, mmol g 1) was determined by the following equation: qe ¼C0 Ce V m (1) where C0 and Ce

3are the initial and equilibrium concentrations of solute in the liquid phase (mmol L1), respectively, m is the mass of adsorbent (g) and V is the volume of solution

(L). For the

pH adsorption edge experiments, the suspension pH was varied from 2 to 11 and adjustment was made byadding 1 N HCl or 1 N KOH solutions. All adsorption runs were replicated twice with averages used as theresults. Batch adsorption experiments – binary solute systems For the binary adsorption experiments, threesynthetic effluents containing amoxicillin and ampicillin were prepared (Table S1 of the ESI†). Adsorptionisotherm experiments were performed in a closed batch system by equilibrating the synthetic effluentscontaining a known amount of O-MMT

11on a reciprocal shaker for 24 h at room temperature. The initial pH of

all effluents ranged between 6 and 7. The equilibrium concentration of remaining antibiotics was determinedspectrophotometrically in a multi-component quantitation mode at two measurement wavelengths of 245.8nm and 252.2 nm. Five mixed samples with pure amoxicillin and ampicillin standards were made toconstruct the calibration curve. The following mathematical formula was used to calculate the equilibriumamount of solutes i and j in the adsorbed phase: C0;i=j Ce;i=j qe;i=j ¼ m V (2) where



41qe,i and qe,j are the equilibrium loading of solutes i and j in the solid phase

(mmol g 1), C0 and Ce refer to the

3initial and equilibrium concentrations of solute in the solution (mmol L 1), m isthe mass of O-MMT used (g) and V is the volume of the effluents (L). Resultsand discussion

Textural properties and surface chemistry of Na-MMT and O- MMT materials The electron micrographs of

19Na-MMT and O-MMT are shown in Fig. 1.

SEM analysis con?rmed that

19Na-MMT and O-MMT are

both crystalline solids of micrometer size. The lump alike morphology of Na-MMT with smooth surfacecharacteristic is clearly seen from Fig. 1a. In comparison, the SEM image of O-MMT displays theagglomerated sheet alike morphological feature and surface roughness (Fig. 1b). N2 adsorption– desorptionisotherms (Fig. S1 of the ESI†) ascertain that both clay sorbents are highly mesoporous with mixed micro-and meso-sized pores. The characteristic type H4 hysteresis loop observed in the relative pressure range of0.4–0.6 is the Fig. 1 SEM images of Na-MMT (a) and O-MMT (b). indication of an adsorption phenomenonof gases typical for complex micro-mesoporous solids, which include micropores ?lling, pore condensationand cavitation-induced evaporation mechanisms.26 The BET speci?c surface area of Na-MMT was 122.2m2 g 1 and this value dramatically fell to 65.8 m2 g 1 of O- MMT. Similarly, total pore volume of Na-MMTwas 0.11 cm3 g 1 while that of O-MMT was 0.06 cm3 g 1 (Table S2 of the ESI†). The decreased

31BET speci?c surface area and pore volume

revealed that some interior adsorption sites became inacces- sible by N2 molecules due to the blocking oflarge surfactant cations within the pores. The pore size distribution curves (inset Fig. S1†) support N2adsorption–desorption results that high percentage of mesopores with a diameter about 3–4 nm exist in Na-MMT and O-MMT. Furthermore, a notable distribution of pore sizes outside the range of 3–4 nm wasobserved in O-MMT, likely due to the surfactant cations loading into the interparticle pores within the ‘house-of-cards’ structure that enlarge the corresponding pore size. This is consistent with other studies dealingwith organoclay preparation employing long alkyl-chain cationic surfactants.27–29 Con?rmation of theorgani?cation of Na-MMT was also shown from elemental analysis results in the ESI Table S3.† In this table,it can be seen that O-MMT contains about 0.83 wt% N and 12.1 wt% C (the C/N ratio is 14.58) where thepresence of carbon and nitrogen atoms in Na-MMT is not observed. Based on the carbon and nitrogen



contents, it can be estimated that each gram of clay contains 0.59 mmol of intercalated C14-trimethylammonium cations. Thermal gravimetric analysis was used to examine the weight loss arising fromorganic content and the related degradation mechanisms. The TGA curves in the ESI Fig. S2† show that theweight loss by about 7% below 150 C corre- sponds to the loss of surface water and water associated withNa-MMT micropore structure. Between 200 C and 400 C, about 0.0063 g H2O per g clay was lost, whichmight be attributed to desorption of water from the interlayer space. Irrevers- ible dehydroxylation of thelayered silicate structure takes place in the temperature range of 600–700 C. The weight change of the claycould be neglected above 700 C. The presence of organic moieties increases the number of decompositionsteps for organoclay. As illustrated in Fig. S2,† the TGA pro?le of O-MMT indicates four weight-loss steps:(I) the loss of water (dehydration) that occurs at 110 C and ends at 150 C; (II) decomposition of the bondedstructure of organic modi?er in the interlayer space at 200–300 C; (III) dehydroxylation of the silicate layersaround 600 C and proceeds till around 700 C and (IV) further decomposition of the organic surfactant at720–800 C. The thermogram pattern of bare MTAB indicates the weight loss at 110 C resulted fromdehydration, followed by structural degradation that occurs at 250–400 C. Finally, the maximumdecomposition takes place around 600 C due to the incomplete oxidation of the organic moieties under N2atmosphere. The spectral characteristics

23of Na-MMT and O-MMT are displayed in the ESI Fig.

S3.† Several infrared absorption bands of Na-MMT were observed at speci?c wavenumbers, which are thetypical of montmorillonitic mineral: 3614 cm 1 of O–H stretching of structural hydroxyl groups located at thesurface and along the broken edges, 3342 cm 1 and 1636 cm 1 of stretching and bending vibrations of OHgroup in water molecules, 1087 cm 1 of Si–O stretching, 936 cm 1 of Al–Al–OH hydroxyl-bending vibration,

33522 cm 1 of Al–O–Si bending vibration and 475 cm 1 of Si–O–Si bendingvibration. The

insertion of organic modi?er (MTA+ cation) into the interlayer spacing gave rise to symmetric andasymmetric sp3 C–H stretching vibrations of methyl and methylene groups at 2900– 2800 cm 1 andsymmetric sp3 C–H bending vibration at 1464 cm 1. It can be shown that the vibrational bands correspondto

6Si–O stretching, Al– Al –OH bending, Al–O–Si

bending and Si–O– Si bending between Na-MMT and O-MMT are essentially iden- tical. This suggests thatthe unit-cell framework of montmorillonite mineral (tetrahedral-octahedral-tetrahedral layered sheets)remains intact during microwave irradiation. On the other hand, the absorption intensities of stretching andbending vibrations of hydroxyl group at 3400 cm 1 and 1600 cm 1 dropped, which might be attributed to theremoval of adsorbed water molecules from the clay lattice. X-ray diffractograms of



23Na-MMT and O-MMT are given in the ESI Fig. S4.† Here, the XRD pattern ofMMT

lump was not shown due to its similar characteristic to that pattern of Na- MMT. A broad (001) re?ection wasnoted at 2-theta of 6.44 for MMT lump and 6.49 for Na-MMT, characterizing the basal spacing ofmontmorillonite. This information suggested that the transformation of MMT lump to Na-MMT through cationexchange did no or little alteration on the mineralogical properties of clay. The occurrence of othercrystalline phases such as quartz, calcite, feldspar and anatase was observed in addition to montmorillonitecrystal planes and basal re?ections. Semi- quantitative mineral analyses of MMT lump and Na-MMT basedon XRD data showed that these clay impurities accounted for 20–22% of total crystalline phases (the purityof smectite phase was 78–80%). The intercalation of MTA+ cations into the montmorillonite structure leadsto the

19expansion of basal spacing from 1. 36 nm to 1.95 nm

and interlayer spacing from 0.39 nm to 0.98 nm. The interlayer spacing was determined by subtracting themeasured basal spacing with the unit-cell thickness of a single tetrahedral-octahedral-tetrahedral layeredsheet of montmorillonite, which is 0.97 nm.30 The structural conformation of the surfactant in the interlayerspacing can be interpreted by considering the magnitude of increased basal spacing and the molecularstructure of intercalated surfactant cations (the thickness of polar ‘head’ and apolar ‘tail’). The loading ofMTA+ cation, a single C14 alkyl chain trimethylammonium surfactant with a concentration of 1.5 times ofthe CEC would result in the pseudotrilayer conformation; that is the alkyl chain of surfactant is packed inparallel to the plane of silicate tetrahedral sheets with interlocked-chains array.27,31 The equivalenceexchange between Na+ and MTA+ cations also renders the clay surface to be organophilic, which is suitableto sorb organic compounds such as antibiotics. Effects of solution pH on the adsorptive removal ofamoxicillin and ampicillin The pH is a crucial factor both towards the surface charge density of adsorbentand the ionic speciation of adsorbate in the liquid phase, which determines the effectiveness of a sorptionprocess. The presence of acid (carboxylic) and base (amino) surface functional groups within amoxicillin andampicillin structure contributes to the amphoteric nature of these drugs, in which these groups are ionisablefollowing the pH changes. As shown in Table 1, amoxicillin has three acid dissociation constants of 2.4(pKa1 of carboxylic), 7.4 (pKa2 of amine) and 9.6 (pKa3 of phenol) while for ampicillin, the pKa values are2.7 (carboxylic) and 7.3 (amine).14 Accordingly, amoxicillin species are mainly as a cation in the acidicsolution (pH < 2.4), a zwitterion between pH 2.4 and 7.4 and an anion in the alkaline solution (pH > 7.4).Similarly, ampicillin exists predominantly in cationic, zwitterionic and anionic forms at pH < 2.7, 2.7 < pH <7.3 and pH > 7.3, respectively (see Fig. S5 of the ESI†). The distribution diagrams showing the percentageof amoxicillin and ampicillin species at room temperature under different solution pHs was presented in theESI Fig. S6.† The in?uence of pH on the adsorbed amount of amoxicillin and ampicillin in single systems isgiven in Fig. 2. In this ?gure, the increasing amount of amoxicillin or ampicillin adsorbed by Na-MMT wasseen with the increase of pH from 2 to 7 and then a progressive decrease on the removal percentage wasencoun- tered at pH above 7. Similar observation was reported by Moussavi et al. for the removal ofamoxicillin from water using NH4Cl-induced activated carbon.32 The limited uptake at low pH, particularlybelow pKa1 of amoxicillin or ampicillin was due to net repulsion between positively charged edge hydroxylsurfaces of montmorillonite crystallites (silanol and aluminol sites) and the cationic adsorbate molecules.



From the speciation diagrams in Fig. S5,† it can be shown that at low pH range (pH 2–3), the cationicamoxicillin and ampicillin accounted for 84% and 86% of total species in the solution, respectively. Theremoval processes of amoxicillin ( 24%) and ampicillin ( 27%) could still take place in the acidic environmentdue to the dipole-induced interaction between protonated amine group and the Si-tetrahedral basal oxygensurface in addition to ion exchange between Na+ interlayer cations and protonated amoxicillin or ampicillin.The latter phenomenon (cation exchange) has been veri?ed to be the sorption controlling mechanism at lowpH in the studies conducted by Jiang et al.33 and Wang et al.34 for the removal of cipro?oxacin usinglayered manganese oxide and Ca-montmorillonite, respectively. With the increase of pH approaching pKa2of the adsorbate, the ratio of zwitterion to cation increases gradually for both antibiotics and this leads to theincrease of removal percentage, Fig. 2 Variation of pHs on the removal of amoxicillin and ampicillin in singlecomponent systems (adsorbent dosage ¼ 1 g L 1, contact time ¼ 24 h, T ¼ 303.15 K). reaching the highestlevel of 63.5% for amoxicillin and 65.2% for ampicillin at near-neutral pH. Under this condition, theprotonated amine in amoxicillin or ampicillin structure might play important for the enhanced removalprocess by forming electric force that attracts this cationic group to deprotonated aluminol (Al–O ) or silanol(Si–O ) edge sites. Furthermore, the concentration of H+ ions that compete with NH3+ group for the basaloxygen surfaces diminished at elevated pH, facilitating the sorption process. The sorption of amoxicillin orampicillin under alkaline environment is unfavorable since both antibiotics exist as an anion (attributed tothe presence of conjugate bases of carboxylic acid and phenol groups) thus electrostatic repulsive forcedominates. Such phenomenon was con?rmed from a gradual decline on the removal percentage from63.5% to 18.7% for amoxicillin and 65.2% to 20.3% for ampicillin as the pH rose from 7 to 11. For theadsorption of amoxicillin or ampicillin using O-MMT, higher removal percentage was observed across the pHrange studied, indicating that O-MMT has superior adsorption capacity. In the acidic solution, about 31–61%of amoxicillin and 33–62% of ampicillin was removed while the removal effi- ciency in highly alkalineenvironments (pH 10–11) ranged between 27% and 41% for amoxicillin and 31–43% for ampicillin. Thehighest removal percentage was obtained at pH around 7, corresponding to 77.8% removal for amoxicillinand 81.3% removal for ampicillin. It can be implied that higher adsorptive removal of O-MMT was mainlyattributed to the exchange of interlayer Na+ cation with MTA+ cation, which provides bene?cial features tothe sorption process. Aside from expanding the interlayer and basal spacing, the intercalation of MTA+cation would expose additional charge-bearing sites for the binding of incoming amoxicillin or ampicillinmolecules. The intercalated MTA+ cation would interact hydrophobically with amoxicillin or ampicillin throughintermolecular attraction forces, possibly van der Waals force or permanent dipole– dipole interactionbetween positively charged nitrogen atom located at the ‘head’ of MTA+ cation and negatively chargedcarboxylate anion (RCOO ) in amoxicillin or ampicillin structure. The forces of electrostatic attraction actingbetween carboxylate anionic groups and positively charged clay' edge sites might also take place.Furthermore, the hydrophobic alkyl chain of the intercalated surfactant played important role by serving as asorption domain for the allocation of organic (antibiotic) molecules.35–37 Thus, it can be argued that theintercalated surfactant cation in the clay' interlayer spacing has a crucial in?uence by forming organicpartition phases with quite different affinities toward the organic groups. Ultimately, there was no linearrelation between the surface properties and adsorption capacity as O-MMT sorbent exhibited higher sorp-tion capacity although its

21speci?c surface area and pore volume were lower compared to the

original material. Modeling adsorption isotherm data for single antibiotic systems The so-called adsorptionisotherm is vital information describing the equilibrium distribution of solute adsorbed on a solid surface to



that of in the liquid with which it is in contact at a given temperature. The single component adsorptionisotherms of amoxicillin and ampicillin were analyzed by Freundlich and Langmuir models. The Freundlichmodel, which originally developed in 1909 for expressing the isothermal variation of a quantity of gasadsorbed by unit mass of adsorbent with pressure, has an empirical mathematical form as follows:38 qe ¼KF C1e/n (3) where KF is the Freundlich constant related to the adsorption affinity

42[(mg g 1) (L mg 1)1/n or (mmol g 1) (L mmol 1)1/n] and n

is a dimensionless intensity factor characterizing the surface heterogeneity degree. Generally,

16the value of constant n is greater than unity and the

adsorption favorability can be eval- uated based on the following n values: favorable (2–10), moderatelydifficult (1–2) and poor (<1).39 Langmuir model is perhaps the simplest and most useful semi-empiricalmodel for interpreting various physical and chemical adsorption phenomena in gas- and liquid-phasesystems as well as many real sorption processes. Langmuir (1918) postulated his isotherm based on thekinetic principle of adsorption of gases on the plane surfaces of ideal solids.40 The mathematical expressionof Langmuir isotherm is equated by the following formula: KLCe qe ¼ qm 1 þ KLCe (4) where qm is theLangmuir constant of maximum sorption capacity when the solid is covered with a monolayer (mg g 1 ormmol g 1), also denotes a practical limiting sorption capacity and assists in the comparison of adsorptionperformance and KL is the adsorption affinity constant (L mg 1 or

36L mmol 1). The essential features of Langmuir isotherm can be expressedin terms of equilibrium parameter

RL, with classi?cations of the adsorption nature: favorable – convex isotherms (0 < RL < 1), unfavorable–concave isotherms (RL > 1), linear (RL ¼ 1) or highly favorable/non-reversible (RL ¼ 0). The RL value canbe calculated using a formula proposed by Weber and Chakravorti:41 RL ¼ 1 (5) 1 þ KLC0

26where C0 is the initial solute concentration in the

liquid phase, which is 0.80

27mmol L 1 for amoxicillin and 0. 82 mmol L 1

for ampicillin. The correlations of adsorption equilibrium data were performed by nonlinear regression ?ttingusing SigmaPlot so?ware (Version 12.3, Systat So?ware Inc.). Fig. 3 displays the ?tting comparisonbetween Freundlich and Langmuir models against single adsorption equilibrium data of amoxicillin andampicillin. The values of the ?tted isotherm parameters are listed in Table 2. At a glance, Langmuirisotherm describes experimental data better than Freundlich isotherm. The favorability of Langmuir model



also visually con?rmed from the isotherm curves that exhibit a convex character. According to system ofclassi?cation of solution adsorption isotherms suggested by Giles and colleagues,42 the isotherm curves atall temperatures belong to a L2-type, which the indicative of solute adsorbed ?at on the surface. Similarisotherm type was found for the immobilization of tetracycline on kaolinite surface18 and adsorption of anantihistamine medicine (chlorpheniramine) using Ca-montmorillonite21 and activated charcoal.43Considering the adsorption affinity, this parameter can rise or fall with temperature change, depending onwhether physisorption or chemisorption that dominates in the sorption system. In this study wherechemisorption controls the adsorption of amoxicillin and ampicillin, the adsorption affinity should be higherwith increasing temperature because higher temperature hastens the movement of solute molecules in theliquid phase to be bound on the surface. Both Freundlich and Langmuir isotherms clarify this behavior, asshown in Table 2. Higher affinity of solute molecules to O-MMT surface is expec- ted because the sorbentpossesses organophilic surface stem- ming from the substitution of MTA+ cation in place of interlayer Na+cation. In terms of adsorptivity, ampicillin was preferen- tially adsorbed to the surface rather than amoxicillin.The next examination is on the parameter qm of Langmuir model, which the indicative of maximummonolayer capacity of a particular sorbent. With the

21increase of temperature from 303 .15 K to 323 .15 K, the maximum monolayer

capacity rose from

0.079 mmol g 1 to 0.090 mmol g 1 for Na-MMT and from 0.143 mmol g 1 to 0.157 mmol g 1 for O-MMT ifampicillin was considered as the solute. The increasing maximum monolayer capacity accompanyingtemperature rise revealed that chemisorption was the controlling phenomenon. The remaining analysis ison the parameter n of Freundlich model, which is a characteristic constant for the surface heterogeneitydegree. It can be seen that there is no clear correlation between this parameter and temperature, as in thesorption systems of amoxicillin/Na-MMT and amoxicillin/O-MMT, the values of n decrease with temperatureincrease while in the ampicillin/ O-MMT system, an opposite trend is obtained. Even, a ?uctu- ating value ofn with temperature was observed in the ampicillin/Na-MMT system. It should also be noted that themagnitude of constant n should be higher for the adsorption on the O-MMT surface. Again, the Freundlichisotherm cannot capture this well. Therefore, it can be concluded that Langmuir isotherm outperformsFreundlich isotherm in describing adsorption equilibria data. In addition, the calculated RL values all con?rmed the favorable

32nature of adsorption process across the studied temperatures. The adsorptioncapacity

of O-MMT was compared with other adsorbents to evaluate its application as a viable alternative to treatamoxicillin and ampicillin from water and wastewater.

26Based on the Langmuir analysis, the maximum adsorption capacity of O-

MMT was estimated to be

0.124– 0.133 mmol g 1 (45.3–48.6 mg



29g 1) for amoxicillin and 0. 143– 0.157 mmol g 1

(49.9–54.9 mg g 1) for ampicillin. A number of studies had used activated carbons as the primary adsor-bent to remove various antibiotics. Ding and co-workers44 prepared the sludge-derived activated carbons toremove a mixture of 11 antibiotics include amoxicillin from diluted water solutions. The estimated totaladsorption capacity of 80–300 mg g was obtained from the Langmuir–Freundlich 1 prediction. Similaradsorption capacity of bare and NH4Cl- induced activated carbons (261.8 vs. 438.6 mg g 1) was reported byMoussavi et al.32 for batch adsorption of amoxicillin. Although the synthesized O-MMT exhibited loweruptake capacity than carbon-based materials, the preparation method of the organoclay is simpler and lessenergy- and time-consuming compared to chemical or thermal activation pathways to synthesize thecarbons. Furthermore, the montmorillonite lump used to prepare the organoclay is a cheap and easilyavailable material in large quantity. The application of organo-bentonite to remove amoxicillin had beeninvestigated by Zha and colleagues.45 However, lower adsorption capacity of the organobentonite (27.85–30.12 mg g 1 at 303.15 K and 313.15 K) was obtained compared to the values reported in this study. The O-MMT sorbent also showed superior adsorption capacity than chitosan beads46 (45.3 vs. 8.71 mg g 1 at pH6.5) for batch amoxicillin removal. Modeling adsorption isotherm data for binary antibiotic systemsMulticomponent adsorption depends on the number of solutes competing for the sorption sites, theirspeciation, Fig. 3 The Freundlich and Langmuir model fittings against single adsorption equilibrium data ofamoxicillin and ampicillin ( 303.15 K, 313.15 K and 323.15 K). concentrations, residence time, etc., which allcontribute to the interference and competition phenomena in the solution and at the solid/solutioninterface.47 O?en, most of available empirical or semi-empirical models lack of theoretical basis and theymay not be able to accurately analyze the system behavior over the entire range of measurements. Notwith-standing the most realistic model so-called the ideal adsorption solution theory (IAST) and its modi?cations,such Table 2 Correlation isotherm

9parameters for adsorption of amoxicillin and ampicillin in single

component systems as fitted with Freundlich and Langmuir models Freundlich parameters Langmuirparameters Adsorbent Adsorbate T (K) KF (mmol g 1) (L mmol 1) n n R2 qm (mmol g 1) KL (L mmol 1) R2RL Na-MMT O-MMT Ampicillin Amoxicillin Ampicillin Amoxicillin 303.15 0.083 313.15 0.089 323.15 0.097303.15 0.058 313.15 0.068 323.15 0.074 303.15 0.151 313.15 0.156 323.15 0.170 303.15 0.129 313.150.132 323.15 0.134 9.54 0.87 15.12 0.82 8.51 0.86 15.86 0.86 13.96 0.85 12.45 0.86 10.22 0.95 10.34 0.9712.53 0.96 15.30 0.94 14.32 0.94 13.03 0.98 0.079 124.81 0.080 158.09 0.090 169.56 0.056 96.68 0.064108.29 0.071 113.46 0.143 141.11 0.146 183.37 0.157 206.44 0.124 122.94 0.126 143.75 0.133 182.58 0.990.98 0.98 0.99 0.98 0.99 0.95 0.94 0.94 0.93 0.97 0.95 0.0092 0.0073 0.0068 0.0124 0.0111 0.0106 0.00820.0063 0.0056 0.0098 0.0084 0.0066 as fast-IAS and real adsorption solution theory have found reasonablesuccess to correlate multicomponent adsorption equilibria data, the complex algorithm for the model solutioninvolving numerical integration at each step of iteration procedure requires the use of advanced computerresources. On the other hand, extended-Langmuir is a simple model with adequate thermodynamic basisand useful insights for analyzing multicomponent adsorption equilibria. To describe the equilibriumcompetitive adsorption in the multicomponent systems, Langmuir model for pure component adsorptionequilibria can be easily extended to the following formulation: qm;iKL;iCe;i qe;i ¼ XN (6) 1 þ KL;iCe;i i¼1where i is the number of components, qm,i and qe,i are the maximum adsorbed amount of each component(mmol g 1) and the adsorbed amount of each component per mass of adsorbent at equilibrium concentration



Ce,i and KL,i is the individual adsorption affinity constant of each component (L mmol 1). For a liquid phasesystem consisting of two components, one can use the extended-Langmuir equation in the form:

18qe;1 ¼ qm;1KL;1Ce;1 1 þ KL;1Ce;1 þ KL;2Ce;2 qe;2 ¼ qm;2KL;2Ce;2 1 þ

KL;1Ce;1 þ KL;2Ce;2

(7) (8) The success

34of the extended-Langmuir model in analyzing binary adsorption equilibrium

data

has been reported in several studies.47–49 The validity of the model was justi?ed based on the simple errorminimization between predicted and measured qe values. The predicted qe values for each component canbe calculated from eqn (7) and (8) by introducing the ?tted parameters qm and KL belonging to singleadsorption data. Such approach, however, does not ?t for correlating binary adsorption data becauseadsorption in the binary system involves interference and competition between solutes in the solution and onthe active surface sites; both these characteristics are negated in the single adsorption system. Therefore,to improve the accuracy of predictions and theoretical sound of the extended-Langmuir model, we propose amodi?cation to each parameter above by addressing competitive adsorption behavior. In the binary system,the active sorption sites are accom- modated by two solutes with a speci?c surface coverage. Therefore, themaximum sorption capacity should be the sum of fraction of surface covered by each solute multiplied by itsmaximum sorption capacity or can be expressed as follows: qm,bin ¼ qm,1(singl)q1 + qm,2(singl)q2 (9)where q1 and q2 are the fractional surface coverage of solute 1 and 2, respectively. Here, we argue thattotal monolayer surface coverage by both solutes should not exceed unity, considering that some surfacesites are inactive for adsorption. The parameter KL measures how strong the solute molecules are attractedto the surface. The higher the affinity, the more solute molecules are adsorbed on the surface. Since thesolute species compete each other, their affinities to the surface should be weaker compared to that ofsingle adsorption system. Accord- ingly, the use of parameter KL from single adsorption data is not valid anda simple theoretical treatment to this parameter is given as follows: KL;1ðbinÞ ¼ KL;1ðsinglÞ 1 q 2 q1 þ q2(10) KL;2ðbinÞ ¼ KL;2ðsinglÞ 1 q1 q1 þ q2 (11) In eqn (10) and (11), the competitive adsorption betweensolutes is expressed as a ratio of fraction of surface covered by one solute to that of total surface coverageby both solutes. Introducing eqn (10) and (11) into eqn (7) and (8) gives the decreased to 0.437 and 0.263with increasing ampicillin following equations: concentration to 50 wt% and 75 wt%, respectively. Likewise,the qm;1ðsinglÞq1 þ qm;2ðsinglÞq2 KL;1ðsinglÞ 1 q 2 Ce;1ðbinÞ qe;1ðbinÞ ¼ q1 þ q2 1 þ KL;1ðsinglÞ 1 q2 Ce;1ðbinÞ þ KL;2ðsinglÞ 1 q 1 Ce;2ðbinÞ q1 þ q2 q1 þ q2 (12) qm;1ðsinglÞq1 þ qm;2ðsinglÞq2KL;2ðsinglÞ 1 q 1 Ce;2ðbinÞ qe;2ðbinÞ ¼ q1 þ q2 1 þ KL;1ðsinglÞ 1 q 2 q 1 q1 þ q2 Ce;1ðbinÞ þKL;2ðsinglÞ 1 Ce;2ðbinÞ q1 þ q2 (13) Eqn (12) and (13) are both called as the modi?ed extended-Langmuir model. Here, q1 and q2 were treated as ?tting parameters with the following constraints: q1 > 0,q2 > 0 and q1 + q2 < 1. The proposed model was ?tted to experimental data by performing computer-aidednonlinear regression ?tting. The accuracy of predictions was assessed from the coefficient of determination(R2) values obtained from the computational results. The binary adsorption equilibrium data are obtained byconducting adsorption experiments at 303.15 K and near- neutral pH with O-MMT as the sorbent andmixtures of amoxicillin and ampicillin as the waste effluents. Here, O-MMT was employed as the sorbent



due to its superior adsorption capacity compared to Na-MMT. Also, the choice of conducting binaryadsorption experiments at pH range of 6–7 was based on the single adsorption results in which at this pHrange, the highest removal of amoxicillin and ampicillin took place. Fig. 4 shows the correlation results ofbinary adsorption equilibrium data ?tted with the modi?ed extended-Langmuir model (represented as wire-mesh plot). The values of q1 and q2 obtained from the model ?tting are given in Table 3. From Fig. 4,

40it can be seen that the modi?ed extended-Langmuir model

can correlate binary adsorption data adequately. In comparison, the ?tness of original extended-Langmuirmodel to the experimental data is given in the ESI Fig. S7.† Noticeably, the modi?ed extended- Langmuirmodel gave better representation than the original model, suggesting that the inclusion of fractional surfacecoverage may improve the accuracy of predictions and assist in interpreting the adsorption behavior inbinary system. The fraction of active sorption sites covered by either amoxicillin or ampicillin

9decreases as the concentration of the opposite adsorbate in the mixtureincreased. For example, the

fraction of adsorption sites covered by amoxicillin for effluent A (75 wt% amoxicillin + 25 wt% ampicillin) was0.639 and this value ampicillin loading on the surface of O-MMT increased with decreasing concentration ofamoxicillin in the mixture. One can also notice that total monolayer coverage on the surface in all systems issimilar, typically about 0.89 (expressed as a fractional quantity). This supports our previous argument thatsome of the surface sites are unavailable for the sorption, likely due to the surfactant blockade and toconsider that the sorbent has a void fraction. With regard to adsorptivity, both amoxicillin and ampicillin areless likely adsorbed on the surface due to competition effect. Consequently, fewer amounts of amoxicillin(44.8%) and ampicillin (48.3%) were removed from a binary mixture (effluent B) compared to single solutesystems for near- neutral pH adsorption. To this end, the modi?ed extended- Langmuir model will turn backinto the original Langmuir model if only one solute component is considered in the sorption system (q2 andCe,2 are zero in eqn (12) and q1 and Ce,1 are zero in eqn (13)). Adsorption thermodynamic Adsorptionthermodynamic relates the equilibrium of adsorption to those properties which cannot be directly measuredfrom the experiment, such as activation energy (Ea, kJ mol 1), the Gibb's

22free energy change (DG , kJ mol 1), standard enthalpy change (DH , kJ mol1), standard entropy change (DS , kJ mol 1 K 1)

and isosteric heat of adsorption (DHx, kJ mol 1). The Gibb's free energy change is an important criterion forspontaneity of a chemical process and it can be related to adsorption equilibrium constant by the followingreaction isotherm: DG0 ¼ RT ln KD (14) where

20R is the ideal gas constant (8.314 J mol K 1), T is the 1 temperature (K) andKD is the linear sorption distribution



Fig. 4 The fitting performance of modified extended-Langmuir model against binary adsorption equilibriumdata of various effluents containing amoxicillin and ampicillin. coefficient, de?ned as a ratio between theequilibrium surface concentration of adsorbed solute and the equilibrium solute concentration in the liquidphase. The value of KD is determined by plotting a straight line of ln (qe/Ce) versus Ce and extrapolating tozero Ce according to Khan and Singh.50 The variation of thermodynamic equilibrium constant withtemperature can be expressed in terms of standard enthalpy change and standard entropy change by theclassical van't Hoff formula: DS0 DH 0 ln KD ¼ R RT (15) A plot of natural logarithm of the thermodynamicequilib- rium constant, ln KD versus the reciprocal temperature, 1/T (Fig. 5)

10will be linear with the slope and y-intersection point giving the values of DH

and

DS , respectively.

16Table 3 The fitted and calculated equilibrium parameters for binary adsorptionof

effluents containing amoxicillin (adsorbate 1) and ampicillin (adsorbate 2)a Fitted parameters Calculatedparameters KL,1(bin) KL,2(bin) qm,bin Effluents q1 q2

28(L mmol 1) (L mmol 1) (mmol g 1)

R2 A 0.639 0.253 88.07 B 0.437 0.455 60.23 C 0.263 0.618 36.70 40.02 0.115 0.97 71.98 0.119 0.98 98.980.121 0.98 a Adsorption temperature ¼ 303.15 K, Adsorbent ¼ O-MMT. Fig. 5 Thermodynamic

3plots of ln KD versus 1/T for the adsorption of

amoxicillin and ampicillin in single component systems ( amoxicillin/Na-MMT; ampicillin/Na-MMT; amoxicillin/O-MMT; ampicillin/O-MMT). Table 4 summarizes the values of thermodynamic parameters for singleadsorption of amoxicillin and ampicillin. The free energy change of adsorption decreased

14with an increase in temperature and this suggests that higher temperaturemakes the sorption process easier. The negative values of

DG con?rm that

8adsorption is thermodynamically feasible and spontaneous with high

preference of solutes to the surface. In this case, the adsorption of



amoxicillin and ampicillin on O-MMT is more likely to take place at a given temperature, as re?ected fromthe magnitude of DG values. The values of DH and DS are 43.8 kJ mol and 162.8

10J mol 1 1 K 1 for

amoxicillin/Na-MMT system and 45.3

10kJ mol 1 and 178.4 J mol 1 for

ampicillin/Na-MMT 1 K system. The positive sign of DH is an indication of the endo- thermicity of adsorptionand suggests the possibility of strong binding between adsorbate and adsorbent. Furthermore, themagnitude of DH values may give an idea whether the adsorption

8belongs to physisorption (2.1–20.9 kJ mol 1) or chemisorption (80–200 kJ mol

1). As shown in Table 4, it seems that the adsorption of amoxicillin and ampicillin on Na-MMT or O-MMT canbe attributed to the combination of physisorption and chemisorption

10rather than a pure physical or chemical adsorption. The

thermodynamic quantity DS is de?ned as

8a measure of randomness in the system. The positive value of

DS re?ects high affinity of solute to the sorption sites, also the

14increased randomness at the solid/solution interface during adsorption

process. Some signi?cant changes in the internal structure of adsorbate andadsorbent may cause the

system to gain extra translational and rotational entropies, such as from the displacement of adsorbed waterby the adsorbate, interlayer cation exchange between Na+ and MTA+ or the partitioning of adsorbatespecies in the hydrophobic alkyl chain of the intercalated surfactant. Furthermore, the positive value of DSrevealed a strong con?nement of solute in the solid phase. Since binary adsorption experiments in this studywere conducted at single temperature (303.15 K), it is not possible to analyze thermodynamic behavior ofthe process. However, one can predict it theoretically, for instance the values of free energy change ofadsorption should be less negative because adsorption is less energetically favorable. Also, the magnitudeof the entropy change might be greater due to increased disorder in the system as a result of competitiveadsorption between the adsorbed components. Batch adsorption tests using real pharmaceutical



wastewater and regenerability evaluation In order to test the feasibility of O-MMT adsorbent for real sorptionapplication, batch adsorption experiments using real pharmaceutical wastewater were conducted at 303.15K for 24 h. The adsorbent dose was ?xed at 10 g L 1. The wastewater was randomly collected from ?vesampling points from a wastewater treatment plant of local pharmaceutical and health care productsmanufacturer at Sidoarjo city, East Java. Prior

15to Table 4 Thermodynamic parameters for adsorption of

amoxicillin and ampicillin in single component systems Adsorbents T (K) Amoxicillin DG DS DH (kJ mol 1) (J

39mol 1 K 1) (kJ mol 1) Ampicillin DG DS DH (kJ mol 1) (J mol 1 K 1)

(kJ mol 1) Na-MMT 303.15 313.15 323.15 O-MMT 303.15 313.15 323.15 5.55 162.8 7.18 8.81 9.92 191.411.84 13.75 43.8 8.78 10.56 12.35 48.1 12.12 14.16 16.19 178.4 203.6 45.3 49.6 adsorption, the collectedsamples were vacuum-?ltered with a Buchner funnel to remove coarse particles. The initial pH of thewastewater ranged between 5.5 and 6. Amoxicillin and ampicillin were detected as two major constituentswith initial concentrations of 0.16 and 0.11 mmol L 1, respectively. Other antibiotics, such as cipro?oxacin,chloramphenicol and cefotaxime were also detected in trace levels (Table S4 in the ESI†). Solution of 0.1M CaCl2 was used as the desorbing agent. Desorption experiments were carried out by mixing antibiotics-loaded O-MMT sorbent with CaCl2 (solution to solid ratio of 100) and the

15mixture was allowed to stand for 24 h under shaking at room temperature. The

removal percentages of each antibiotic as a function of regeneration cycle are displayed in Fig. 6. Thehighest removal percentage was observed for amoxicillin, followed by ampicillin, chloramphenicol, cipro- ?oxacin and cefotaxime with total removal percentage of 68.2%. A gradual decrease in the removalpercentage of each antibiotic was noticed as the regeneration cycles progressed; likely due to the inability ofdesorbing solution to completely detach bound antibiotics from the organoclay surface. Similar observationwas obtained by Wu et al.51 for desorption studies of cipro- ?oxacin from kaolinite and montmorilloniteclays. By the end of the ??h cycle, total removal percentage of all antibiotics by O-MMT was 49.5%. Theseresults show that O-MMT is a promising sorbent for economical and effective treatment of real effluentscontaining antibiotic compounds. Furthermore, O-MMT sorbent possesses good adsorptive retention sincetotal removal percentage was about 50% even a?er ?ve successive adsorption–desorption cycles.Desorption of antibiotics from the organoclay surface might be due to the ligand-promoted dissolution ofmetal-antibiotic surface complexes between Ca2+ and the zwitterionized antibiotics through the involvementof carboxylic ( COO ) groups.51,52 FT-IR spectroscopy study of O-MMT sorbent a?er desorption by CaCl2solution (see Fig. S3 in the ESI†) supported this argument in which the intensities of absorption peaksassociated with C]O stretching of carboxylic Fig. 6 Removal percentages of various antibiotics in realpharmaceutical effluents and reusability tests of O-MMT adsorbent ( amoxicillin; ampicillin;chloramphenicol; ciprofloxacin; cefotaxime). group at 1700 cm 1 and N–H stretching and bending vibra-tions of amine



30group at 3400 cm 1 and 1600 cm 1

diminished. In addition, the stretching peaks correspond to sp3 C–H bond in CH3 and CH2 groups at2900–2800 cm 1 remain unal- tered a?er Ca2+ desorption, indicating that cation exchange between MTA+in the interlayer spacing and Ca2+ was not feasible to take place, due to the latter cation possesses loweraffinity than the former toward the clay surface. Conclusions Organo-montmorillonite (O-MMT) with highadsorption capacity and organophilic surface has been successfully prepared by microwave-assistedirradiation of aqueous suspension containing Na-montmorillonite (Na-MMT) and MTAB cationic surfactant.The synthesized O-MMT showed potential applications for detoxifying amoxicillin and ampicillin in singleand binary systems. Analysis of adsorption equilibrium data for single antibiotic systems revealed thatLangmuir model outperformed Freundlich model. The proposed

34extended-Langmuir model with the inclusion of surface coverage wassuperior to

original model in representing binary adsorption equilibrium data. The proposed model can also adequatelycapture theoretical insights of binary sorption behavior. Thermodynamically, the adsorption of amoxicillin andampicillin on Na-MMT/O-MMT was energetically favorable/ spontaneous (DG < 0) and endothermic (DH > 0)with high preference of adsorbed solutes toward the surface (DS > 0). The regeneration study revealed thefeasibility of O-MMT sorbent to be efficiently reused for ?ve cycles of adsorption–desorption in handling realpharmaceutical wastewater containing multiple antibiotic compounds. References 1 R. N. Brogden, A.Carmine, R. C. Heel, P. A. Morley, T. M. Speight and G. S. Avery, Drugs, 1981, 22, 337–362. 2 J. P. Hou andJ. W. Poole, J. Pharm. Sci., 1971, 60, 503–532. 3 H. Schmitt and J. Rombke, in Pharmaceuticals in theEnvironment: Sources, Fate, Effects and Risks, ed. K. Kummerer, Springer Berlin Heidelberg, Berlin, 2008,pp. 285–303. 4 M. V. Walter and J. W. Vennes, Appl. Environ. Microbiol., 1985, 50, 930–933. 5 L.Guardabassi, D. M. A. Lo Fo Wong and A. Dalsgaard, Water Res., 2002, 36, 1955–1964. 6 M. L.Richardson and J. M. Bowron, J. Pharm. Pharmacol., 1985, 37, 1–12. 7 T. Coskun, H. A. Kabuk, K. B.Varinca, E. Debik, I. Durak and C. Kavurt, Bioresour. Technol., 2012, 121, 31–35. 8 S. Larcher and V.Yargeau, Appl. Microbiol. Biotechnol., 2012, 96, 309–318. 9 R. Andreozzi, M. Canterino, R. Marotta and N.Paxeus, J. Hazard. Mater., 2005, 122, 243–250. 10 I. A. Balcioglu and M. Otker, Chemosphere, 2003, 50,85–95. 11 K. A. Rickman and S. P. Mezyk, Chemosphere, 2010, 81, 359– 365. 12 S. Z. Li, X. Y. Li and D. Z.Wang, Sep. Purif. Technol., 2004, 34, 109–114. 13 Z. Qiang, J. J. Macauley, M. R. Mormile, R. Surampalliand C. D. Adams, J. Agric. Food Chem., 2006, 54, 8144–8154. 14 E. S. Elmolla and M. Chaudhuri, J.Hazard. Mater., 2010, 173, 445–449. 15 E. S. Elmolla and M. Chaudhuri, Desalination, 2011, 272, 218–224.16 X. Xiao, R. P. Hu, C. Liu, C. L. Xing, X. X. Zuo, J. M. Nan and L. S. Wang, Chem. Eng. J., 2013, 225,790–797. 17 M. Miyata, I. Ihara, G. Yoshid, K. Toyod and K. Umetsu, Water Sci. Technol., 2011, 63, 456–461. 18 Z. H. Li, L. Schulz, C. Ackley and N. Fenske, J. Colloid Interface Sci., 2010, 351, 254–260. 19 W.Yan, J. F. Zhang and C. Y. Jing, J. Colloid Interface Sci., 2013, 390, 196–203. 20 N. Liu, M. x. Wang, M. m.Liu, F. Liu, L. P. Weng, L. K. Koopal and W. f. Tan, J. Hazard. Mater., 2012, 225–226, 28–35. 21 Z. H. Li, P.H. Chang, J. S. Jean, W. T. Jiang and H. L. Hong, Colloids Surf., A, 2011, 385, 213–218. 22 S. M. Mitchell,J. L. Ullman, A. L. Teel and R. J. Watts, Sci. Total Environ., 2014, 466–467, 547–555. 23 H. Xu, W. J.Cooper, J. Jung and W. Song, Water Res., 2011, 45, 632–638. 24 S. J. Chipera, G. D. Guthrie and D. L.Bish, Rev. Mineral. Geochem., 1993, 28, 235–249. 25 Y. Yang, Y. Chun, G. Y. Sheng and M. S. Huang,



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